Speckle reduction method and apparatus

ABSTRACT

An apparatus adapted for confocal imaging of a non-flat specimen comprising a coherent light source for producing a light beam, imaging optics adapted to focus the light beam into at least one spot on a surface of a specimen, and a detector adapted to receive and detect light reflected from the specimen surface. The imaging optics comprise at least one optical component located so that the light reflected from the specimen surface passes therethrough on its way to the detector. The optical component is movable so as to move the at least one spot, within a range of movement, to a number of distinct locations in a plane perpendicular to the apparatus&#39; optical axis, within the detector&#39;s integration time.

FIELD OF THE INVENTION

This invention relates to the reduction of speckle noise in opticalsystems comprising imaging optics, in which a coherent light source isused.

BACKGROUND OF THE INVENTION

A common difficulty associated with the use of coherent light sourcessuch as lasers in imaging optical systems is a phenomenon known asspeckle. Speckle arises when coherent light scattered from a roughsurface is detected by an intensity detector that has a finite aperture,such as an observer's eye or a detector. The image on the screen appearsto be quantized into little areas with sizes equal to the detectorresolution spot. The detected spot intensity varies randomly fromdarkest, if contributions of the scattering points inside the spotinterfere destructively, to brightest if they interfere constructively.This spot-to-spot intensity fluctuation is referred to as speckle. Theresultant speckled light signal on the detector appears as spatial andtemporal noise in whatever sensor is used in the imaging system.

Speckle reduction is known to involve averaging a number of independentspeckle configurations, i.e. obtained from different un-correlated andnon-interfering reflecting beams. Since speckle depends on essentiallythree light parameters: angle, polarization, and wavelength of theilluminating laser beam, independent speckle configurations can begenerated through the diversification of any of these three lightparameters. To solve the problem of speckle, many attempts have beenmade, mostly based on angle diversification, obtained by means ofdiffusers and/or movable optical elements, or by means of polarizationdiversification.

In U.S. Pat. No. 4,155,630 to Ih, there is disclosed a process andapparatus for improving image creation in a coherent light imagingsystem which involves directing a diffused light onto a mirror having arocking motion whereby angle diversification is obtained. The rockingmotion causes the reflected rays to sweep a two-dimensional area andfocus the reflected light through a diffuser before collimating the raysfor use in image creation. Applying a combination of voltages to threeindependent piezo-electric crystals upon which the mirror is mountedproduces the rocking motion of the mirror.

U.S. Pat. No. 6,081,381 to Shalapenok, et al., describes a method andapparatus for eliminating speckle in an optical system by anglediversification obtained by the use of a diffuser and by a rotatingmicro-lens array having a rotational speed related to the laserparameters. The micro-lens illumination comes off of a stationarydiffuser and eventually provides a large area that is uniform andspeckle free illumination.

U.S. Pat. No. 4,511,220 to Scully, discloses a laser target speckleeliminator for laser light reflected from a distant target whoseroughness exceeds the wavelength of the laser light. The apparatusincludes a half-plate wave member, a first polarizing beam splittermember, a totally reflecting right angle prism, and a second polarizingbeam splitter member, all of which are in serial optical alignment. Usedin combination, the components convert a linearly (i.e., vertically)polarized laser light beam having a known coherence length, into twocoincident, orthogonally polarized beams that are not coherent with eachother. The resultant beams have an optical path difference exceeding theknown coherence length of the laser, thereby eliminating the speckle inthat system.

In U.S. Pat. No. 6,577,394 to Zavislan, there is disclosed a scanninglaser confocal microscopy system for reducing speckle from scatterersthat exist outside (above and below) the section which is being imagedby utilizing orthogonally polarized sheared beams. The sheared beams arefocused to spots that are laterally or vertically offset. The polarizedbeams have opposite senses of circular polarization.

SUMMARY OF THE INVENTION

In accordance with the present invention there is provided a method andapparatus for speckle reduction in an imaging system using coherentlight, particularly useful for determining the surface profile of anon-flat object/specimen by confocal imaging. To perform such imaging,the apparatus typically comprises a confocal aperture and means forfocusing an incident beam at a plurality of axial locations. In suchimaging, also known as confocal microscopy, speckle is particularlyproblematic because the confocal imaging process requires focusing laserlight on the specimen surface when the most speckle occurs.

Thus, the apparatus of the present invention comprises a coherent lightsource for producing a light beam, imaging optics adapted to focus thelight beam into at least one spot on a surface of a specimen, and adetector having an integration time, adapted to receive and detect lightreflected from the surface; the imaging optics comprising at least oneoptical component located so that the light reflected from the specimensurface passes therethrough on its way to the detector, the opticalcomponent being movable so as to move the at least one spot to a numberof distinct locations in a plane perpendicular to the optical axiswithin the detector's integration time.

The method of present invention for the confocal imaging a non-flatspecimen comprises:

providing an apparatus comprising a source of coherent light and adetector;

focusing the coherent light into at least one spot on a surface of thespecimen by means of imaging optics comprising a movable opticalcomponent;

directing light reflected by the surface toward the detector via themovable optical component;

detecting the light by the detector; and

moving the movable optical component so as to move the at least one spotto a number of distinct locations within the integration time of thedetector.

The movement of the optical component is such that a distance betweentwo spot locations that are maximally remote from each other does notexceed the lateral resolution of the apparatus.

The lateral resolution of the apparatus is the minimum lateral distancebetween two adjacent points on the specimen for which the apparatus candistinguish a difference in height.

Due to the specific location of the movable optical component of thepresent invention which ensures that both the incident and reflectedlight passes therethrough, the detector does not feel the movement ofthe optical component, i.e. the detected image is static.

During the movement of the optical component as defined above, the spotis moved from one location to another. This results in obtaining anumber of independent speckle configurations corresponding to the numberof the distinct locations of the spot, which are averaged by thedetector over its integration time.

The confocal imaging apparatus of the present invention preferablycomprises a beam-splitter and the imaging optics include a collimatinglens and an objective lens, where at least the objective lens isdisposed between the beam-splitter and the specimen.

The movable optical component referred to above may be the objectivelens itself or an additional element located between the beam splitterand the specimen. Such additional element may be a transparent wedge ora mirror.

The movement of the optical component may be regular or irregular. Oneexample of the regular movement of the optical component is one thatcauses the spot on the specimen surface to follow a circular path aroundthe location of the center of the spot if the optical component werestatic. A circular movement of the objective lens may accomplish thiscircular path, i.e. the center of the lens moves in a circle about theoptical axis.

The invention may be applied equally well to multi-spot confocal systemssuch as in a confocal scanning apparatus disclosed in the Applicant'spublication WO 00/08415. There, the laser light beam is divided into aplurality of beams to obtain a plurality of spots on the specimensurface. In such apparatus, the movable optical element in accordancewith, the present invention, will move each of the spots in the mannerdescribed above.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carriedout in practice, preferred embodiments will now be described, by way ofnon-limiting example only, with reference to the accompanying drawings,in which:

FIG. 1 is schematic view of a confocal scanning system as known in theart;

FIG. 2 is schematic view of a confocal scanning apparatus according toan embodiment of the present invention;

FIG. 3 is schematic view of a confocal scanning apparatus according to adifferent embodiment of the present invention;

FIG. 4 is schematic view of a confocal scanning according to a furtherembodiment of the present invention; and

FIG. 5 is schematic view of a confocal scanning according to stillfurther embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a typical apparatus for determining a 3-D profile, ortopography, of the surface of an object/specimen, e.g. a tooth, at adesired lateral and axial resolution. The apparatus is a confocalimaging system comprising a laser 10, which constitutes a source ofcoherent light 12; a collimating lens 14 disposed in front of the laserfor collimating the emitted light into a beam 16; a beam splitter 18through which the collimated beam 16 passes; an optical imagingcomponent in the form of an objective lens 20 for focusing the lightbeam into a beam 17 (hereinafter ‘incident light beam’), on a non-flatspecimen 22 whose topography is to be determined. The above componentsare disposed along an optical axis A. The specimen 22 is shown in aperspective view to emphasize that it contains a depth (in Z-directioncoinciding with the optical axis A) as well as a length and a width (inan X-Y plane perpendicular to the optical axis A). The incident lightbeam 17 that illuminates specimen 22 and forms thereon a spot 38, isreflected back through lens 20, producing a reflected beam 19 whichpasses through the lens 20 towards the beam splitter 18. The apparatusfurther comprises an image detector 30 having an integration time, and afocusing lens 26 and a confocal aperture or pinhole 28 disposed betweenthis detector and the beam splitter so that the beam 19 is reflected bythe beam splitter 18 towards the detector 30 passes through the focusinglens 26 and the pin-hole 28.

When the specimen 22 is scanned axially (Z-axis), either by axialdisplacement of the specimen or by axial displacement of the objectivelens 20, it will take positions at which the incident light beam 17 willor will not be focused on its surface. In the latter case, the reflectedlight 19 will be partially blocked by the pinhole 28 thus producing alow intensity signal at the detector 30. As the specimen 22 gets closerto an in-focus position, the amount of light passing through the pinhole28 increases, yielding a maximum signal from the detector 30 at the bestfocus.

The intensity of the signal is thus related to the depth (i.e. along theZ-axis) of a scanned point. By imaging at a number of depths(Z-coordinates) an intensity profile can be obtained, which is known asan. Optical Section Profile (OSP) 34. The peak of the OSP 34 yields therelative depth, or position, of the surface point on the specimen beingscanned. Repeating the depth scanning process for e very X and Ylocation on the specimen surface yields a full 3-D profile, ortopography, of the specimen.

The phenomenon of speckle in the reflected light results in a noisy OSP34, seen as wavy lines 36 in FIG. 1, impairing the accuracy of the depthcoordinate determination. The nearer to focus the scanning spot 38 is onthe specimen 22, the stronger the speckle contrast becomes, hence thenoise recorded by the detector 30 is more significant at the peak of theOSP 34 where it is most unwanted.

FIG. 2 illustrates a first embodiment of the present invention, where inan apparatus similar to that shown in FIG. 1, the objective lens 20 hasan associated movement mechanism 40 for producing movement of the lens,as indicated by arrows 42 and 44. A possible movement mechanism 42 maybe piezo-electric actuator.

The movement of the objective lens 20 is in a periodic manner so thatthe same path made by a spot 48 on the specimen is repeated at a certainrate. In this path, the spot 48 is moved so as to visit distinctlocations within an area 46 of the specimen 22. This path may have anyshape, e.g. be circular, oval, square, rectangular, polygonal,non-regular, etc. Spot trace 47 in FIG. 2 is an example of the footprintof a circular path taken by the spot 48 produced by the movement of theobjective lens 20. A circular movement of the objective lens 20 mayaccomplish this circular path, i.e. the center of the lens moves in acircle about the optical axis A.

The length of the path of the spot 48 is preferably as large aspossible, to provide a greater number of independent speckleconfigurations, corresponding to the number of locations, for maximumstatistical sampling. However, the distance between the most remote spotlocations during the spot's movement shall be smaller than the lateralresolution of the apparatus. The lens movement is synchronized to theintegration time of the detector 30 such that the averaging of theindependent speckle configurations may be performed over one fill periodof spot movement, or a portion of it.

The detector 30 averages out these independent speckle configurations,thereby yielding a relatively smooth OSP 49, as shown in FIG. 2. Theaveraged signals collected during the integration time may be recordedautomatically by known means and will not be discussed further.

The activities described above should be repeated to produce arelatively smooth OSP 49 at each scanned point, to determine thespecimen's roughness, or topography.

FIG. 3 illustrates an apparatus in accordance with another embodiment ofthe present invention, which is similar to the apparatus described abovewith reference to FIG. 2, with the difference being in that it includesa movable optical element in the form of a transparent wedge 50, madefor example of glass, disposed between the beam splitter 18 and theobjective lens 20, with the latter being static. The wedge 50 isrotatable to stir the incident beam 17 in an angle, thereby giving riseto a corresponding movement of the spot 38 on the specimen 22 beingscanned, as discussed in connection to FIG. 2.

It should be understood that the wedge 50 is merely an example of arefracting optical element whose rotation can produce a suitable anglediversification of the incident beam 17. Clearly, such element may haveany other appropriate shape.

FIG. 4 shows an apparatus according to a further embodiment of thepresent invention, which is similar to the apparatus described abovewith reference to FIG. 2, with the difference being in that it comprisesa rotatable reflecting optical element in the form of a mirror 60(preferably a dual axis mirror) located between the beam splitter 18 andthe objective lens 20, with the latter being static.

The movement of the mirror 60 moves the collimated incident beam 17 inangle thereby giving rise to a corresponding movement of the laser spoton the specimen 22.

A fundamental virtue of the apparatus of the present invention is that,although there are moving optical components (e.g. the objective lens20, the wedge 50 and the mirror 60) causing a spot to move accordinglyon the specimen 22, there is no loss in the imaging (or confocalmeasurement) quality. The detector 30 does not observe the motion of thelight spot on the specimen 22 since the reflected beam 19 passes backthrough the moving optical component (objective lens 20, wedge 50 andmirror 60). Thus, the spot on the detector remains a stationarydiffraction limited spot.

FIG. 5 illustrates a further embodiment of the present invention whereina multi-spot parallel confocal system is used, of the kind disclosed inApplicant's publication WO 00/08415. In this system, a grid or spotarray 70 illuminates the specimen 22 (shown as a tooth in FIG. 5) andeach spot 70 n of the array axially scans the specimen to produce arelatively smooth OSP 49 n of the corresponding X-Y area 46 n on thespecimen 22. In other words, each illuminating spot 70 n in the array 70undergoes a depth scan.

As seen in FIG. 5 a single laser beam 72 is collimated and passes into amicro-lens array 74 comprising a plurality of micro-lenses 74 n. Thearray 74 generates spots at the focal points of the micro-lenses 74, onespot per micro-lens, correspondingly producing the desired spot array70. The spot array 70 is directed onto the specimen 22, via a beamsplitter 75 using magnifying optics including a source objective lens 76and a specimen objective lens 78. The reflected light is directed, viathe same lenses 76 and 78 and the beam splitter 75, toward a detector 82having an array of n detector elements and having n pinholes 80corresponding to the micro-lenses 74 n of the micro-lens array 74.

A relatively smooth OSP 49 n is generated from each detector element ofthe detector array 82, and thus the Z-coordinate is determined, at eachcorresponding X-Y area 46 n. Again., the confocal scanning is obtainedby moving the specimen objective lens 78 along the Z-axis over thedesired depth of scan.

Any of the speckle reduction embodiments described hereinabove withreference to FIGS. 2 to 4, may be applied to the multi-spot confocalapparatus.

It can be appreciated that the above-described speckle reductionapparatus and method can be realized in a variety of embodiments andthat those described hereinabove are merely examples. For example, otheroptical components may be suitable for moving an incident beam on aspecimen in order to reduce speckle. Further, the above mentionedcomponents may be used in combination with each other—or with otheroptical components.

1. An apparatus adapted for confocal imaging of a non-flat specimen,said apparatus having an optical axis and a predetermined lateralresolution and comprising a coherent light source for producing a lightbeam, imaging optics adapted to focus the light beam into at least onespot on a surface of a specimen, and a detector having an integrationtime and adapted to receive and detect light reflected from saidsurface; said imaging optics comprising at least one optical componentlocated so that the light reflected from the specimen surface passestherethrough on its way to the detector, said optical component beingmovable so as to move the at least one spot, within a range of movement,to a number of distinct locations in a plane perpendicular to theoptical axis, within said integration time of the detector, wherein adistance between the most remote said distinct locations is smaller thansaid lateral resolution.
 2. The apparatus according to claim 1, whereinthe moving optical component is an objective lens.
 3. The apparatusaccording to claim 2, wherein the objective lens is adapted to movecircularly about the optical axis.
 4. The apparatus according to claim1, wherein the moving optical component is a reflecting optical element.5. The apparatus according to claim 4, wherein the reflecting opticalelement is designed to move on dual axes.
 6. The apparatus according toclaim 1, wherein the moving optical component is a non-imaging opticalelement.
 7. The apparatus according to claim 6, wherein the movingoptical component is a generally wedge-shaped transparent component. 8.The apparatus according to claim 7, wherein the transparent component ismade of glass.
 9. The apparatus according to claim 7, wherein thetransparent component is rotatable about the optical axis of theapparatus.
 10. The apparatus according to claim 1, wherein the movingoptical component is designed to produce a circular spot pattern on thespecimen.
 11. The apparatus according to claim 1, wherein the light beamis composed of an array of light beams.
 12. The apparatus according toclaim 1, wherein the apparatus further comprises a beam-splitter.
 13. Anapparatus according to claim 1, wherein said apparatus is adapted foraxially scanning a surface of the said specimen and for obtaining adepth measurement of said surface in a direction substantially parallelto said optical axis.
 14. A method for confocal imaging of a non-flatspecimen, the method comprising: providing an apparatus comprising asource of coherent light and a detector; focusing the coherent lightinto at least one spot on a surface of the specimen by means of imagingoptics comprising a movable optical component; directing light reflectedby the surface toward the detector via the movable optical component;detecting the light by the detector; and moving the movable opticalcomponent so as to move the at least one spot to a number of distinctlocations within the integration time of the detector, wherein adistance between the most remote said distinct locations is smaller thana lateral resolution of said apparatus.
 15. A method according to claim14, wherein the movable optical component moves on dual axes.
 16. Amethod according to claim 14, wherein the movable optical componentrotates about an optical axis of the apparatus.
 17. A method accordingto claim 14, wherein the movable optical component produces a circularspot pattern on the specimen.
 18. A method according to claim 14,further comprising the step of axially scanning a surface of the saidspecimen and obtaining a depth measurement of said surface in adirection substantially parallel to an optical axis of said coherentlight incident on said surface.